Plasma torches of non-transferred arc type, an example of which is shown in FIG. 1, comprise two coaxial tubular electrodes 1, 2 separated by a plasma-generating gas 3 injection chamber 4, devised in such a way that this gas is injected vortex-fashion into the torch. The electrodes are dubbed upstream and downstream with respect to the direction of the gas flow. The downstream electrode 2 is supplemented with a starter point 21.
The manner of operation of the torch is briefly recalled hereinbelow:                The application of a continuous electric current to the terminals of the electrodes makes it possible to initiate an electric arc 10 between the electrodes 1, 2. At present, the triggering of this electric arc inside high-power (that is to say greater than 80 kW) plasma torches, is carried out by “contact” short-circuiting the two electrodes: the upstream electrode 1, movable in translation along its axis, advances inside the plasma torch until it touches the downstream electrode 2. The two electrodes are thereafter fiercely parted until separated by an operating distance d, at the same time as a current is applied to the terminals of the two electrodes thus creating the electric arc 10.        On contact of the arc, the gas 3 heats up rapidly and is transformed into a very high-temperature plasma.        The injection of plasma-generating gas 3 into the torch coupled with the use of a field coil 5 thereafter makes it possible to stretch this arc so that it takes a position inside the electrodes of the torch.        The ionized gas is expelled in tandem therewith through the downstream end of the torch. This results in a plasma jet or “dard” at very high temperature typically 4000 K.        The magnetic field coil 5, wound around the upstream electrode, makes it possible to drive the movement of the arc foot 10 so as to control the region of wear and to increase the longevity of the electrode 1.        A cooling system 6 ensuring circulation of a water film in contact with the exterior surface of the electrodes allows the cooling of the components exposed to the arc or to the plasma.        The operating point of the torch is chosen by simultaneously fixing a specific electric current and by controlling the flowrate of plasma-generating gas introduced into the torch.        
Although this ignition process is very efficacious, it nonetheless exhibits drawbacks.
In particular, the upstream electrode must be mounted on a hydraulic ram allowing its translation. This ram together with the associated hydraulic plant represents a sizable investment and requires regular maintenance operations.
Adjustment of the two time lags between the retreat of the electrode and the application of the electric current is very tricky.
Moreover the electrical power supplies must be capable of withstanding high no-load voltages as well as large variations in current, typically of the order of 100 A depending on the priming current between the voltage spike required during the “contact” short-circuit and the operating voltage. They must make it possible to support the current in the electrical circuit composed of the two electrodes during the transition between the contact short-circuit and the open circuit, the latter comprising the two electrodes and ionized plasma-generating gas.
Other techniques are also known for triggering an electrical discharge in a low-power plasma torch, such as the use of a conventional sparkplug or an RF discharge. But having regard to the small separation between the electrodes that is required for these techniques, they are not suited to energy transfer systems such as high-power plasma torches.